1. Field of the Invention
The present invention relates generally to Multimedia Broadcast/Multicast Service (MBMS). More particularly, the present invention relates to a method of informing User Equipment (UE) in a CELL_Dedicated CHannel (DCH) state of Preferred Frequency Layer (PFL) information when a session starts in Frequency Layer Convergence (FLC) being the process where UE is requested to preferentially select the frequency layer on which the desired MBMS service is intended to be transmitted.
2. Description of the Related Art
Owing to today's development of communication technology, Wideband Code Division Multiple Access (WCDMA) mobile communication systems are evolving beyond traditional voice service toward multimedia broadcasting/communications to transmit a large amount of data in packet service and multimedia service. To support multimedia broadcasting/communications, MBMS is under consideration which provides services from one or more multimedia data sources to multiple UEs.
MBMS is a service for transmitting the same multimedia data to a plurality of recipients over a wireless network. Because recipients of an MBMS service share one radio channel, radio transmission resources are saved. MBMS services enable transmission of multimedia data including real-time video and audio data, still images, and text. Multimedia transmission requires a large amount of radio transmission resources. Since an MBMS service transmits the same data to a plurality of cells where users are located, it is provided in a Point-to-Point (PTP) or Point-to-Multipoint (PTM) mode depending on the number of users in each cell.
FIG. 1 schematically illustrates nodes involved in providing an MBMS service in a mobile communication network. The MBMS service is provided in a 3rd Generation Project Partnership (3GPP) system based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), compliant with the 3G asynchronous mobile communication standards, by way of example.
Referring to FIG. 1, UE components 161, 162, 163, 171 and 172 are MBMS-enabled terminals or MBMS subscribers. First and second cells 160 and 170 (Cell 1 and Cell 2) are physical or logical service areas under control of Node Bs which transmit wireless MBMS data to subscribers. A Radio Network Controller (RNC) 140 controls the plurality of cells 160 and 170 and selectively transmits MBMS data to a particular cell. It also controls radio channels established for MBMS services. The RNC 140 is connected to UEs 161 to 172 via Radio Resource Control (RRC) interfaces.
The RNC 140 is connected to a Packet-Switched or Packet Service (PS) network (not shown) such as the Internet through a Serving GPRS Support Node (SGSN) 130. Communications are conducted between the RNC 140 and the PS network by PS signaling. The connection between the RNC 140 and the SGSN 130 is called an Iu-PS interface. The SGSN 130 controls MBMS services for each subscriber. The main functions of the SGSN 130 include management of service billing data for each subscriber and selective transmission of multimedia data to the RNC 140.
A transit network 120 provides a communication path between a Broadcast Multicast-Service Center (BM-SC) 110 and the SGSN 130. The transit network 120 can be connected to an external network through a Gateway GPRS Support Node (GGSN: not shown). The BM-SC 110, which is an MBMS data source, is responsible for scheduling MBMS data.
The RNC 140 is connected to a Circuit-Switched (CS) network (not shown) through a Mobile Switching Center (MSC) 150. The CS network is a legacy connection-based voice-oriented communication network. Communications are conducted between the RNC 140 and the MSC 150 by CS signaling. The connection between the RNC 140 and the MSC 150 is called an Iu-CS interface.
MBMS data streams from the BM-SC 110 are delivered to UEs 161 to 172 through the transit network 120, the SGSN 130, the RNC 140, and Node Bs/cells 160 and 170.
While not shown, a plurality of SGSNs may participate in one MBMS service and a plurality of RNCs may exist for each SGSN. Each SGSN selectively transmits data to the RNCs and each RNC selectively transmits to cells. For this purpose, the SGSN and the RNC store lower-layer node lists (an RNC list for the SGSN and a cell list for the RNC) in which the nodes are to receive MBMS data.
MBMS systems support FLC technology for allocating a PFL to each available MBMS service and requesting UEs to preferentially select to the frequency layer on which their desired MBMS service is intended to be transmitted. FLC information, that is, PFL information indicates a PFL per MBMS service.
FIG. 2 illustrates an exemplary cell configuration in an MBMS system when FLC technology is applied.
Referring to FIG. 2, three cells 221, 222 and 223 (Cell 1, Cell 2 and Cell 3) using different frequencies, Frequency 1, Frequency 2 and Frequency 3, are spatially overlapped. Before an MBMS service session starts, a plurality of UEs 211 to 214 camp on cells 221, 222 and 223. In the illustrated case of FIG. 2, frequency 2 of Cell 2 is set as the PFL of the MBMS service.
A Core Network (CN) 201 covers a BM-SC, a transit network, and MSC and an SGSN. CN 201 is connected to first and second RNCs 202 and 203 (RNC 1 and RNC 2) that manage cells 221, 222 and 223. Wired links 231 between the CN 201 and RNCs 202 and 203 are Iu interfaces, and wired link 232 between RNCs 202 and 203 is an Iur interface. While not shown, Node Bs exist between the RNC 203 and UEs 211 to 214 and directly manage the UEs 211 to 214. The RNC 203 is a Drift RNC (DRNC) that directly manages cells 221, 222 and 223 having the UEs 211 to 214, while the RNC 202 is a Serving RNC (SRNC) that controls radio resources for the UEs 211 to 214.
Typically, UE in a CELL_DCH state where a dedicated channel is established for the UE does not read an MBMS Control CHannel (MCCH) delivering FLC information. Therefore, when the UE moves from one cell to another it has no knowledge of the PFLs of MBMS services (hereinafter UE-joined services), which the UE joined and of which the sessions have started. As a result, seamless MBMS service reception is impossible.